X-ray diffraction was used to study variations in the crystallinity of wood and the average thickness and length of the crystallites of cellulose as a function of the number of the year ring in Norway spruce [Picea abies (L.) Karst.]. The crystallinity increased from ring 4 to ring 10 from the pith and was constant after ring 10. The crystallinity of mature wood was about 30% ± 5%. The average thickness and average length of the crystallites were 3.2 ± 0.1 nm and 28 ± 2 nm, respectively; and no systematic variation of these values with the number of the year ring was observed. The mean microfibril angle decreased near the pith but was constant in the mature wood.

The progress of xylem formation in Norway spruce (Picea abies (L.) Karst.) was measured during one growing season in southern Finland. Stem radius was monitored continuously with band dendrometers, and the formation of new tracheids was determined by examination of small increment cores taken twice weekly. Tracheid production started in June and ceased in August. Xylem formation was fastest in early July, when 0.75–1.25 new tracheids were formed per day. The rate of xylem formation was significantly correlated with mean daily temperature. Synchronous fluctuations in tracheid and lumen diameters were observed at the same relative positions within each annual ring, but no relationship existed between the diameters and weather variables. The timing of changes in stem radius differed from the timing of actual xylem formation. Stem radius increased in April and May, and the fastest daily increments were recorded in June. Increases in stem radius slowed in July, but small increases were measured more than a month after xylem formation had ceased. Daily changes in stem radius were correlated with daily precipitation, reflecting changes in stem water content. Therefore, dendrometers are of dubious value for measuring the timing of actual xylem formation. Small increment cores proved to be useful in assessing actual xylem formation, but the method is laborious.

The effect of fertilization on wood density, fibre length, fibre diameter, lumen diameter, proportion of cell wall area, and cell wall thickness of Norway spruce (Picea abies (L.) Karst.) were studied in a nutrient optimization experiment in northern Sweden. On the fertilized plots, all essential macronutrients and micronutrients were supplied in irrigation water every second day during the growing season. After 12 years' treatment, data were collected from 24 trees (40 years old) on the fertilized and control plots. Fertilization increased radial growth more than threefold, especially earlywood width, and decreased wood density by over 20% at 1.3 and 4 m height. The decrease in wood density was closely related to the proportion of latewood. The absolute wood density also decreased across the whole annual ring but proportionately more in latewood than in earlywood. A close relationship was found between the wood density and fibre properties, especially with the proportion of cell wall in a cross section of each annual ring, as well as with fibre and lumen width. The absolute cell wall thickness was clearly less related to wood density. However, rather large variations were found between individual trees in the relationship between wood density and fibre properties.

We studied the variation in average wood density of annual rings, earlywood density, and latewood density in addition to ring width and latewood percentage within Norway spruce (Picea abies (L.) Karst.) stems from the pith to the bark, and from the stem base towards the stem apex. Moreover, the variation in wood density within annual rings was studied at the different heights and radial positions in the stem. The material consisted of 85 trees from central and south-eastern Finland. Variation between the annual rings accounted for 11–27% of the total variation in wood density. Only small differences (3–6%) in wood density were found between different heights in the stem. The largest (49–80%) variation in wood density was found within the annual rings. The difference in wood density between earlywood and latewood was smaller in the rings near the pith than in the outer rings. The increasing wood density from the pith outwards was related to increasing latewood density and latewood percentage, whereas the earlywood density increased only slightly from the pith outwards. In a given cambial age (i.e., given rings from the pith), the average wood density of annual rings increased with increasing stem height. In contrast, in the rings formed in the same calendar years (i.e., given rings from the bark), the average wood density of annual rings decreased with increasing stem height. The results of this study verify our knowledge of wood density variation and can further contribute to creating models to predict wood density.

The effect of thinning intensity on growth and wood density in Norway spruce (Picea abies (L.) Karst.) was investigated in two long-term thinning experiments in southeastern Finland. The stands were approaching maturity, and their development had already been studied for 30 years. The intensities of thinning were low, normal, and high (i.e., the stand basal area after the thinning was, on average, 40, 27, and 24 m²·ha^–1, respectively, in Heinola, and 30, 28, and 17 m²·ha^–1 in Punkaharju, respectively). Compared with the low thinning intensity, the normal and high thinning intensities increased the basal-area increment of individual trees by 52% and 68%, respectively. Normal and high thinning intensities resulted in a relatively small reduction (1%–4%) of mean ring density compared with low thinning intensity. The random variation in wood density between and within trees was large. About 27% of the total variation in wood density was related to variation between rings. Our results indicate that the prevailing thinning intensities in Norway spruce stands in Fennoscandia cause no marked changes in wood density. At least, the possible reduction in wood density is low compared with the increase in individual tree growth.